Chemistry 2100 Chapter 15

Enantiomers Enantiomers: Enantiomers: Nonsuperposable mirror images. –As an example of a molecule that exists as a pair of enantiomers, consider 2-butanol.

Enantiomers

chiral asymmetric alanine

plane of symmetry achiral glycine

Enantiomers chiral Objects that are nonsuperposable on their mirror images are chiral (from the Greek: cheir, hand). –They show handedness. The most common cause of enantiomerism in organic molecules is the presence of a carbon with four different groups bonded to it. stereocenter –A carbon with four different groups bonded to it is called a stereocenter.

Drawing Enantiomers

* * Enantiomers matter! *

The R,S System Because enantiomers are different compounds, each must have a different name. –Here are the enantiomers of the over-the-counter drug ibuprofen. –The R,S system is a way to distinguish between enantiomers without having to draw them and point to one or the other.

The R,S System The first step in assigning an R or S configuration to a stereocenter is to arrange the groups on the stereocenter in order of priority. –Priority is based on atomic number. –The higher the atomic number, the higher the priority.

R,S Priority of Some Groups

The R,S System To assign an R or S configuration: 1. Assign a priority from 1 (highest) to 4 (lowest) to each group bonded to the stereocenter. 2. Orient the molecule in space so that the group of lowest priority (4) is directed away from you. The three groups of higher priority (1-3) then project toward you. 3. Read the three groups projecting toward you in order from highest (1) to lowest (3) priority. R S 4. If reading the groups is clockwise, the configuration is R. If reading them is counterclockwise, the configuration is S.

Properties of Enantiomers

Optical activity

plane-polarized light

Fig. 15-6, p. 435

(±) or (dl) optically inactive raceme levorotatory (l) (–) rotation "left-hand isomer" dextrorotatory (d) (+) rotation "right-hand isomer" Assigning Rotation

Multiple Chiral Centers n 2 n For a molecule with n stereocenters, the maximum number of possible stereoisomers is 2 n. –We have already verified that, for a molecule with one stereocenter, 2 1 = 2 stereoisomers (one pair of enantiomers) are possible. –For a molecule with two stereocenters, a maximum of 2 2 = 4 stereoisomers (two pair of enantiomers) are possible. –For a molecule with three stereocenters, a maximum of 2 3 = 8 stereoisomers (four pairs of enantiomers) are possible, and so forth.

2R,3S 2S,3R 2R,3R 2S,3S The Four Stereoisomers of a Simple sugar

Fischer Projections

Threose and Erythrose, Fischer-ized

meso COOH Tartaric Acid

meso COOH

meso Meso form

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**

Stereochemical Reactions

* pyruvic acidlactic acid

* pyruvic acid lactic acid

(–) (+)

(–) (+)

(–) (+)

(–) (+)

(–) (+)

(–) (+)

(–) (+)

(–) (+)

But, Biologically…

(+)







  

  

H+H+   

H+H+   

Chirality of Biomolecules –Because interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer or one of its diastereomers elicit different physiological responses. –As we have seen, (S)-ibuprofen is active as a pain and fever reliever, while its R enantiomer is inactive. –The S enantiomer of naproxen is the active pain reliever, but its R enantiomer is a liver toxin!